Preference of carbon absorption determines the competitive ability of algae along atmospheric CO concentration.

Although many studies have focused on the effects of elevated atmospheric CO on algal growth, few of them have demonstrated how CO interacts with carbon absorption capacity to determine the algal competition at the population level. We conducted a pairwise competition experiment of sp., , and . The results showed that when the CO concentration increased from 400 to 760 ppm, the competitiveness of increased, the competitiveness of sp. and decreased, and the competitiveness of was always the lowest. We constructed a model to explore whether interspecific differences in affinity and flux rate for CO and HCO could explain changes in competitiveness between algae species along the gradient of atmospheric CO concentration. Affinity and flux rates are the capture capacity and transport capacity of substrate respectively, and are inversely proportional to each other. The simulation results showed that, when the atmospheric CO concentration was low, species with high affinity for both CO and HCO (HCHH) had the highest competitiveness, followed by the species with high affinity for CO and low affinity for HCO (HCLH), the species with low affinity for CO and high affinity for HCO (LCHH) and the species with low affinity for both CO and HCO (LCLH); when the CO concentration was high, the species were ranked according to the competitive ability: LCHH > LCLH > HCHH > HCLH. Thus, low resource concentration is beneficial to the growth and reproduction of algae with high affinity. With the increase in atmospheric CO concentration, the competitive advantage changed from HCHH species to LCHH species. These results indicate the important species types contributing to water bloom under the background of increasing global atmospheric CO, highlighting the importance of carbon absorption characteristics in understanding, predicting and regulating population dynamics and community composition of algae.